Mixing speaker sizes in a cab

I built an amp that has an 8" and 10" speaker, but I use that for guitar. I think 8" speakers emphasize too much high end, which for a harp can be problematic (being a bit tinny already). There's a reason why the Fender Bassman with its 4 x 10" layout is so popular among harp players (for larger venues). So my opinion would be that you wouldn't like the harp sound out of a 2 x 8" + 1 x 10". The 2 x 10" would be a better configuration. The idea would be to "beef up" the harp.

Just a reminder, too: Make sure you think about your speaker connections to your amp when building mutiple speaker cabs. Serial vs Parallel will need to be considered when selecting your speaker impedance, and also to provide for the security of your output transformer (the most expensive part of your circuit) in the event of speaker failure.

Speaking of speaker selection, you might take a look at Weber (https://www.tedweber.com/). They have a special "H" configuration for harmonica players which applies a larger dust cap to the speaker cone. They purportedly dampen the high frequency response, and to my ear, they do.

"A question appears to me : what is the difference (tone wise) between a cab with 1x10" and a cab with 2x10" ?"

So much goes into that to make it a tough call from afar. 2x10" will certainly be louder than a 1x10", and you do get more air movement with the added surface area, but because you're splitting the signal between the two -- and both are equal size and if they are the same speaker make and model -- I wouldn't expect much tonal difference. You might hear a bit more foundational lower end than a single, but again, I think loudness will be the prevailing difference.

There is a phenomenon with multi-speaker cabinets called phase cancellation. This results from the small differences or imperfections in the speaker build that causes the sound to be become more "rounded" due to it reaching the ear or recording device at slightly different times. Two speakers would be somewhat subtle. Four or more would enhance that characteristic. Phase cancellation can sometimes be beneficial for harp players as it can "fatten" the tone.

Some builders will put speakers of different makes and models into their cabinets to play with phase cancellation to get a tonal quality they like. This would be to put, say, a 10" Jensen P10R and a Weber 10A125 in the same cabinet, thereby taking advantage of the differences in winding, build, materials, magnets, etc. to get an unusual mix. I've heard it, and it does make a difference. Mixing and matching drivers are an expensive proposition, however, as you have to try different combos to settle on one you like. If you have a bunch of amps laying around like I do, try cannibalizing a few and try them.

For the rest of your search, you're looking more at the cabinet construction as to how it molds the sound quality. Soft, solid wood cabinets work best for our needs, with solid pine having the best resonance. I'd stay away from any manufactured, particle-type boards, although multi-laminate Baltic birch makes an excellent baffle board, and even the cabinet itself. Bare wood has a certain quality in tone, and covered (tolex, tweed, etc.) has one that's slightly different. For the speaker baffle, you need to consider whether you want a floating baffle or a fixed one. Both have their tonal differences.

All of the above (and more) have impact on your tone quality in an amp once the signal leaves the OT. You'll have to experiment to find what works best for your ear (and your wallet).

A last note on the impedance discussion: It’s great that you’re keeping that in mind. I would recommend connecting the speakers in parallel, as that will give you a diving chance to shut off your amp in the event of a speaker failure before the OT fails. With serial wiring, when the speaker blows, so does your OT.

Another neat trick: If you are using a 1/4” phone jack to connect your speakers to your amp circuit, install a switched jack that will switch the center pin connection to ground when the speaker plug is removed. This will save your OT in the event that your speakers are not plugged in or the cable gets inadvertently yanked out.